1. Field of the Invention
The present invention relates generally to electric circuit protection devices, and particularly to protection devices which periodically self check for simulated fault conditions.
2. Technical Background
Electric systems used to supply AC power to residential, commercial, and industrial facilities typically include a breaker panel that is configured to receive power from a utility source. The breaker panel distributes AC power to one or more branch electric circuits disposed in the facility. The electric circuits transmit AC power to one or more electrically powered devices, commonly referred to in the art as load circuits. Each electric circuit typically employs one or more electric circuit protection devices. Examples of such devices include ground fault circuit interrupters (GFCIs), arc fault circuit interrupters (AFCIs), or both GFCIs and AFCIs. Further, AFCI and GFCI protection may be included in one protective device.
A protective device may be conveniently packaged in a receptacle that is configured to be installed in a wall box. The receptacle includes input terminals configured to be connected to an electric branch circuit, i.e., the receptacle includes a hot line terminal and a neutral line terminal for connection to a hot power line and a neutral power line, respectively. The receptacle includes output terminals configured to be connected to a load circuit. In particular, the receptacle has feed-through terminals that include a hot load terminal and a neutral load terminal. The receptacle also includes user accessible plug receptacles connected to the feed through terminals. Accordingly, load devices equipped with a cord and plug may access AC power by way of the user accessible plug receptacles.
When a fault condition is detected, the protection device eliminates the fault condition by interrupting the flow of electrical power to the load circuit by causing interrupting contacts to break the connection between the line terminals and load terminals. As indicated by the name of each respective device, an AFCI protects the electric circuit in the event of an arc fault, whereas a GFCI guards against ground faults. An arc fault is a discharge of electricity between two or more conductors. An arc fault may be caused by damaged insulation on the hot line conductor or neutral line conductor, or on both the hot line conductor and the neutral line conductor. The damaged insulation may cause a low power arc between the two conductors and a fire may result. An arc fault typically manifests itself as a high frequency current signal. Accordingly, an AFCI may be configured to detect various high frequency signals and de-energize the electrical circuit in response thereto.
On the other hand, a ground fault occurs when a current carrying (hot) conductor contacts ground to create an unintended current path. The unintended current path represents an electrical shock hazard. Further, because some of the current flowing in the circuit is diverted into the unintended current path, a differential current is created between the hot/neutral conductors. As in the case of an arc fault, ground faults may also result in fire. A ground fault may occur for several reasons. If the wiring insulation within a load circuit becomes damaged, the hot conductor may contact ground, creating a shock hazard for a user. A ground fault may also occur when the equipment comes in contact with water. A ground fault may also be caused by damaged insulation within the facility.
As noted above, a ground fault creates a differential current between the hot conductor and the neutral conductor. Under normal operating conditions, the current flowing in the hot conductor should equal the current in the neutral conductor. Thus, GFCIs typically compare the current in the hot conductor(s) to the return current in the neutral conductor by sensing the differential current between the two conductors. The GFCI may respond by actuating an alarm and/or interrupting the circuit. Circuit interruption is typically effected by opening the line between the source of power and the load.
Another type of ground fault may occur when the load neutral terminal, or a conductor connected to the load neutral terminal, becomes grounded. This condition does not represent an immediate shock hazard. Under normal conditions, a GFCI will trip when the differential current is greater than or equal to approximately 6 mA. However, when the load neutral conductor is grounded the GFCI becomes de-sensitized because some of the return path current is diverted to ground. When this happens, it may take up to 30 mA of differential current before the GFCI trips. This scenario represents a double-fault condition, i.e., when both the hot conductor and the load neutral conductor are grounded, the GFCI may fail to trip, causing a user to experience serious injury or death.
Accordingly, it is desirable to provide a protection device that is capable of self-testing for both the grounded hot fault condition and the grounded neutral fault condition. In one approach that has been considered, a GFCI is configured to include a timer that initiates a periodic self test of the GFCI. Alternatively, the GFCI initiates a periodic alarm to alert the user to manually push the test button on the GFCI. One drawback to this approach is that the circuitry is relatively expensive and increases the size of the GFCI circuitry.
In another approach that has been considered, a GFCI includes a visual indicator adapted to display a miswire condition. If the hot power source conductor and the neutral power source conductor are inadvertently miswired to the load terminals of the GFCI, the visual indicator is actuated to display the miswire alarm condition. Those of ordinary skill in the art will understand that a miswire condition of this type will result in a loss of GFCI protection at the duplex receptacles on the face of the GFCI. One drawback to this approach is that the GFCI does not include a self-test of the electrical circuit. Another drawback to this approach is that the visual display does not indicate a lock-out of load side power by the interrupting contacts. As such, the user is obliged to correctly interpret and take action based on appearance of the visual indicator.
In yet another approach that has been considered, a GFCI is configured to self-test the relay solenoid that opens the GFCI interrupting contacts when a fault condition is sensed. However, the self-test does not include a test of the electrical circuit.
In yet another approach that has been considered, the self-test is configured to detect the failure of certain components, such as the silicon controlled rectifier (SCR). If a failure mode is detected, the device is driven to a lock-out mode, such that power is permanently de-coupled from the load.
In light of all of the approaches discussed above, there are many other types of failures, such as those involving the GFCI sensing circuitry, that require manual testing. Of course, manual testing requires a user to push the test button disposed on the GFCI. If a fault condition is present, the GFCI trips out after the test button is pushed. This prompts the user to reset the GFCI. If the device fails to reset, the user understands that the device has failed and is in a lock-out condition. This approach has drawbacks as well. While regular testing is strongly encouraged by device manufacturers, in reality, few users test their GFCIs on a regular basis.
Therefore, there is a need for a protection device that is configured to self-test internal device components. There is a further need for a GFCI that is adapted to self-test for both the grounded hot fault condition and the grounded neutral fault condition. There is also a need for a self-testing GFCI which performs self-testing every half-cycle, during a time period when the SCR tripping mechanism does not conduct. There is yet another need for a self-testing device that self-tests without generating false tripping. Finally, a need exists for a self-testing protection device that is characterized by noise immunity.